I think I've got through to 4 or 5 copies of Heroes of Might and Magic 3 now, it'd always just vanish after awhile.
Might and Magic 6, 7 and 8 I have had to buy multiple times due to lending them to people and them either returning broken or not returning.
Dungeon Keeper 3 times, though one is the GoG version.
Neverwinter Nights + Expansions twice each after losing Shadows of Undrentide and the standard disc and getting a newer system.
Halo: Combat Evolved, loved the game, sold my xbox, bought a new xbox just for Halo and Kingdom Under Fire Heroes.
Theme Hospital, lost the first disc I had, cracking game though and I had to get it again.
Sonic The Hedgehog, I owned two copies of it anyway, but sold them both with a megadrive and now have both again.